This invention relates to an acceleration detector and more particularly to an acceleration detector for detecting; knocking in an internal combustion engine.
FIGS. 1 to 4 illustrate one example of a conventional acceleration detector to which the present invention can be applied. This acceleration detector is attached, when in use, to an internal combustion engine for detecting vibration or knocking of the engine.
The acceleration detector comprises a housing 1 defining an annular cavity 2 therein and an acceleration transducer assembly 3 disposed within the cavity 2. The housing 1 comprises a tubular bushing 4 having a through hole 5 and a flange 6. The housing 1 also comprises a ring shaped outer case 7 bonded to the flange 6 of the bushing 4 so that the cavity 2 is defined therein. The outer case 7 also has a connector 8 radially outwardly extending from the outer case 7 so that an output terminal 9a (FIG. 1) and a ground terminal 9b (FIG. 2 can extend through the connector 8 for taking out an output signal from the acceleration transducer assembly 3 disposed within the cavity 2. The acceleration transducer assembly 3 includes a washer-shaped terminal plate 10 connected to the ground terminal 9b through a lead 10a integrally extending from the terminal plate 10. The terminal plate 10 is placed on the flange 6 of the bushing 4. The acceleration transducer assembly 3 further includes an annular piezoelectric element 11 placed on the terminal plate 10, a washer-shaped terminal 12 including a lead 12a connected to the output terminal 9a, an electrically insulating washer 13 disposed on the washer terminal 12, an annular inertial weight 14 placed on the insulating washer 13 and a threaded ring-shaped stop nut 15 thread-engaged with the thread 4a on the tubular bushing 4. The insulating washer 13 may be made of a sheet of polyethylene terephtalate (PET), polyphenylene sulfite (PPS) or the like. An electrically insulating tape or tube 16 is placed on the tubular bushing 4 so that the acceleration transducer assembly 3 is insulated from the bushing 4 even when the washer terminals 10 and 12 as well as the piezoelectric element 11 are eccentrically assembled.
In order to resiliently support and protect the acceleration transducer assembly 3 within the cavity 2 from undesirable environmental conditions, the remaining space of the cavity 2 of the housing 1 which is not occupied by the acceleration transducer assembly 3 is substantially filled with a resilient filler material 17 of a thermo-setting resin. The filler material 17 must be sufficiently resilient after it is cured to allow the movement of the inertial weight 14 relative to the housing 1 when an acceleration is applied to the inertial weight 14 so that the piezoelectric element 11 generates a voltage signal proportional to the pressure exterted on it by the relative movement of the inertial weight 14 against the piezoelectric element 11.
When in use, the acceleration detector is securely mounted on an internal combustion engine (not shown) by a bolt (not shown) inserted into the central through hole 5 of the housing 1. The acceleration or the vibration of the internal combustion engine produces the movement of the inertial weight 14 relative to the housing 1, which causes the piezoelectric element 11 to be stressed by the inertial weight 14, whereby an electrical signal indicative of the movement of the inertial weight 14 relative to the engine is generated from the piezoelectric element 11. The electrical signal is provided through the washer terminal 12, the lead 12a and the output terminal 9a to be analyzed to determine as to whether or not a knocking signal which generates upon knocking of the internal Combustion engine is involved. When it is determined that a knocking signal is contained in the electrical signal, the operating parameters for operating the engine can be adjusted to increase the output power or decrease the fuel consumption rate.
Since the acceleration detector above described utilizes a stop nut 15 thread-engaged with the thread portion 4a of the tubular bushing 4 of the housing 1, the bushing 4 and the acceleration transducer assembly 3 must be held stationary so as not to be rotated together with the stop nut 15 when the stop nut 15 is turned on the cylindrical bushing 4 of the housing 1 to be securely tightened against the acceleration transducer assembly 3 during assembly.
While the outer case 7 of the housing 1 which has a projection in the vicinity of the connector 8 can be held by any suitable tool during the turning of the stop nut 15, a special care must be taken not to damage the outer case 7 which is usually made of a relatively soft thermo-setting resin. Also, since the outer case 7 is attached to the bushing 4 by a bonding agent, the bonding agent between the outer case 7 and the bushing 4 must receive the stress when a turning force is applied to the bushing 4 from the stop nut 15 and the outer case 7 is held stationary.
Further, the tightening rotation of the stop nut 15 can cause the rotation of the inertial weight 14, the insulating washer 13, the washer terminal 12 and the like of the acceleration transducer assembly 3 relative to the housing 1 due to the friction between these components. This may cause the misalignment of terminal lead 12a in the circumferential direction relative to the output terminal 9a. If the misalignment of the terminal lead 12a is too large, the electrical connection between the terminal lead 12a and the output terminal 8 becomes impossible. In such case, the stop nut 15 must be loosened, the washer terminal 12 must be turned to put the terminal lead 12a into the correct position, and the stop nut 15 must be tightened again with a great care and hope so that no misalignment occurs this time. This results in an extended assembly time, leading to an increased cost of the acceleration detector.
This undesirable rotation of the terminal washer 12 cannot be prevented by simply firmly holding the terminal lead 12a against circumferential movement by a suitable tool or a jig, because the terminal leads 10a and 12a are both as thin as from 0.1 mm to 0.2 mm and has a rigidity insufficient to mechanically support the rotational torque applied from the stop nut 15 to the terminal washer 12. If the terminal lead 10 or 12a is firmly held by a tool and the stop nut 15 turned, the terminal lead 10 or 12a as well as the terminal plate 10 or 12 can be easily deformed or damaged by the stress applied thereon from the stop nut 15.
It is also desirable to provide a mechanical positioning means, which also mechanically isolates two terminal leads, in the detector in order to ensure an easy assembly of the detector and a proper electrical connection between the terminal and the leads.
Further, during assembly of the acceleration transducer assembly 3, the bottom end of the insulating tube 16 may sometimes be raised from the inner surface of the flange 6 of the bushing 4, and the inner periphery of the washer terminal 10 is inserted therebetween to be positioned in an eccentric relationship relative to the bushing 4 and to the piezoelectric element 11 as illustrated in FIG. 4. In this position, the washer terminal 10 is eccentric to the piezoelectric element 11 so that a portion of the piezoelectric element 11 is unevenly in contact with the washer terminal 10, resulting in an inaccurate output signal supplied from the acceleration transducer assembly 3.
Also, since the outer diameter of the stop nut 15 is smaller than the outer diameter of the inertial weight 14, the inertial weight 14 cannot be contacted by the stop nut 15 at the entire contacting surface. Therefore, the inertial force of the weight 14 does not effectively act on the piezoelectric element 11 and the output signal generated by the piezoelectric element 11 is small for the mass of the weight 14.